Compared to an incandescent bulb, a fluorescent lamp has higher efficiency and a longer life. On that account, the fluorescent lamp has come into wide use in terms of the global environmental protection and cost effectiveness. In recent years, a bulb-shaped fluorescent lamp with no electrodes has received attention as an economical light source and is now in increasing demand because its life is several-fold longer than a conventional bulb-shape fluorescent lamp with electrodes.
Also, a bulb-shaped fluorescent lamp that is designed to include a fluorescent lamp and a lighting circuit in one piece has also received attention as an energy-saving light source used in housing spaces, hotels, restaurants, and so forth. One of the reasons for the widespread use is that the bulb-shaped fluorescent lamp can be easily set and used just the way a conventional bulb has been used. The bulb-shaped fluorescent lamp has been developed for an electrode-less fluorescent lamp as well as an electrode fluorescent lamp.
As the bulb-shaped fluorescent lamp has become in wide use, there is a new need for the lamp to have a dimming capability like the incandescent bulb. A more specific explanation is given. In hotels or housing spaces, for example, people enjoy a variety of activities, such as reading and having a good time with their families. For each activity, they wish to have an appropriate light intensity so as to stay in a comfortable lighting environment. As such, a bulb-shaped fluorescent lamp having a dimming capability is required.
The light intensity of a bulb can be easily changed simply by adjusting a supplied amount of electricity because the bulb does not emit light by discharging. On the other hand, unlike the bulb, the fluorescent lamp emits light by discharging, and therefore it is difficult to realize a fluorescent lamp with a dimming capability whose functional level is high enough to be practically used.
Recently, responding to user needs for a fluorescent lamp whose light intensity can be adjusted using an existing dimmer designed for a bulb, a bulb-shaped fluorescent lamp with electrodes has been developed. The adjustment in the light intensity of this lamp can be achieved by connecting the lamp to a dimmer designed for a bulb. An example of such is disclosed in Japanese Laid-Open Patent Application No. 11-111486. However, an electrode-less bulb-shaped fluorescent lamp with a dimming capability has not developed so far.
While developing the electrode-less bulb-shaped fluorescent lamp with the dimming capability, the Applicant found the following problems. Suppose to employ an electrode-less discharge lamp lighting apparatus that adjusts the light intensity of the lamp by changing the ratio of lighting-up and lighting-off periods while bringing a turn-on of the alternating current voltage that has been phase-controlled by a dimmer designed for a bulb in sync with an on timing of a lighting-up by intermittent drive. Hereinafter, alternating and direct currents are respectively referred to as “AC” and “DC” in the present specification. Using this electrode-less discharge lamp lighting apparatus, due to an operating delay of an inverter circuit, or a DC/AC converter, the on timing of the lighting-up period sometimes lags behind the turn-on of the phase-controlled AC voltage. Because of this time lag, even after the dimmer is activated by the turn-on of the phase-controlled AC voltage, a current equal to or smaller than a holding current of a triac included in the dimmer is caused upon the turn-on of the AC voltage. As a result, the triac is reopened and the phase-controlled AC voltage is transformed, in other words, the dimmer malfunctions. The detailed explanation regarding this problem will be given below in the preferred embodiments of the present invention.
When this malfunction happens, flicker and no lights, i.e., discontinuous luminous change may occur to the bulb-shaped discharge lamp, making a user uncomfortable. To be more specific, using an incandescent bulb, the user is not bothered with such a problem because the luminous change of the incandescent bulb is continuous even when adjusting the light intensity. However, once having used the incandescent bulb, the user will become sensitive to the discontinuous luminous change of the bulb-shaped electrode-less fluorescent lamp and will experience considerable discomfort.
To solve this problem, the object of the present invention is to provide an electrode-less discharge lamp lighting apparatus, a bulb-shaped electrode-less fluorescent lamp, and a discharge lamp lighting apparatus which suppress discontinuous luminous change by preventing a malfunction from occurring to a dimmer.